1. Field of the Invention
The present invention relates to a dielectric filter, a dielectric duplexer, and a communication apparatus using the same for use, preferably in the microwave band.
2. Description of the Related Art
A known dielectric filter comprises a plurality of dielectric resonators found in a single dielectric block. Each resonator is defined by a through hole extending through the dielectric block and has an inner electrode formed thereon forming a plated through hole. Each plated through hole has an open circuit end and a closed circuit end. Adjacent resonators are capacitively or inductively coupled to one another. Such filters are typically used in microwave-band communication apparatus, etc. In this type of dielectric filter, an attenuation pole is produced at a low-frequency region of the pass band when adjacent resonators are capacitively coupled with each other, and an attenuation pole is produced at a high-frequency region of the pass band when adjacent resonators are inductively coupled with each other.
One such prior art resonator is disclosed in Japanese Unexamined Patent Application Publication No. 7-254806. In this resonator each plated through hole has a stepped portion in the middle thereof to make the axial spacing between adjacent plated through holes at the short circuit side of the plated through different from that at the open side circuit thereof to provide inductive or capacitive coupling therebetween.
FIGS. 12A, 12B and FIG. 13 show this type of dielectric filter. A substantially rectangular dielectric block 1 has three plated through holes 2a, 2b, and 2c formed therein. An outer conductor 3 is formed on the exterior surface of the dielectric block 1 and is directly coupled to the inner conductor of the through holes on the short circuit side thereof. In FIGS. 12A and 12B, the axial spacing between the plated through holes 2a and 2b at the open circuit side is wider than at the short circuit side to create an inductive coupling at area Sab which is stronger than capacitive coupling at area Oab, resulting in inductive coupling between resonators defined by the plated through holes 2a and 2b. Likewise, the axial spacing between the plated through holes 2b and 2c at the open circuit side is wider than at the short circuit side to make the inductive coupling at area Sbc stronger than capacitive coupling at an area Obc, resulting in inductive coupling between resonators formed of the plated through holes 2b and 2c. 
In FIG. 13, the axial spacing between the plated through holes 2a and 2b at the short circuit side is narrower than at the open circuit side to increase inductive coupling at area Sab compared to the capacitive coupling at area Oab, so that resonators defined by the plated through holes 2a and 2b are strongly inductively coupled with each other. On the other hand, the axial spacing between the plated through holes 2b and 2c at the open circuit side is narrower than at the short circuit side to increase capacitive coupling at area Obc compared to inductive coupling at area Sbc, so that resonators formed of the plated through holes 2b and 2c may be capacitively coupled with each other.
In a dielectric filter having three resonators, as shown in FIGS. 12A, 12B, and 13, however, if the second resonator is closer to the first resonator at the short circuit side (so that the first and second resonators are strongly inductively coupled with each other) then the second resonator is farther from a third resonator at the short circuit side, thereby providing weak inductive coupling between the second and third resonators. If the inner diameter of plated through holes increases at the short circuit side in order to enhance the mutual capacitance at the short circuit side between the first and second resonators and between the second and third resonators, the self-impedance of the resonators decreases to reduce the mutual capacitance, thereby canceling out the effect of increasing the inductive coupling. This also decreases the ratio (step ratio) of the inner diameter at the short circuit side to the open side of a plated through hole, thus lowering the wavelength constant, while requiring longer plated through holes, resulting in a more bulky device.
For these reasons, the first and second resonators are inductively coupled with each other, and the second and third resonators are also inductively coupled with each other, which limits the amount of coupling.
Accordingly, it is an object of the present invention to provide a dielectric filter, a dielectric duplexer, and a communication apparatus using the same, which are capable of providing strong inductive coupling among three adjacent dielectric resonators in a dielectric block.
To this end, in one aspect of the present invention, a dielectric filter includes:
A dielectric filter comprising:
a dielectric block having first and second opposed surfaces;
an outer conductor formed on exterior surfaces of said dielectric block;
is a plurality of plated through holes extending from the first to the second surface of the dielectric block;
a respective inner conductor formed on each of the plated through holes, each conductor having a short circuit end directly coupled to the outer conductor and an open circuit end capacitively coupled to the outer conductor; and
at least one of the plated through holes branching into a plurality of legs each having a short circuit end directly coupled to the outer conductor.
With this structure, for example, the axial spacing between the central plated through hole and plated through holes adjacent thereto may be narrowed at the short circuit side. This makes it possible to strongly inductively couple the first and second resonators on the one hand and the second and third resonators on the other, thereby readily achieving a bandpass characteristic of a broad band.
The location at which the plated through hole branches into a plurality of legs is preferably positioned substantially at the center of the plated through holes in the longitudinal direction thereof. This makes it possible to place the inner conductors on adjacent plated through holes close to each other at a high-magnetic-field-strength region, resulting in the maximum amount of coupling between adjacent resonators to achieve a high versatility in design. Furthermore, each plated through hole has a larger cross-section at the open circuit side than at the short circuit side, that is, each has a stepped structure, thereby improving the effect of reducing the axial length of plated through holes.
In another aspect of the present invention, a dielectric duplexer includes a transmission filter and a reception filter, each filter comprising the above-described dielectric filter. The dielectric duplexer allows adjacent resonators to be strongly inductively coupled with each other in a sequential manner, thus achieving a predetermined filter characteristic with a compact chassis.
In another aspect of the present invention, a communication apparatus includes the above-described dielectric filter. Therefore, a compact communication apparatus having a superior communication performance is achieved.